Fix wasm ssh in nix build

[SuperSandro2000]

That are the wasm ssh bits from NixOS/nixpkgs#538802

[pullfrog]

✅ No new issues found.

Reviewed changes — align the Nix build with the Docker/local build layout for Browser SSH WASM assets and drop the obsolete source-path rewrite.

• Move WASM assets to public/ in nix/package.nix — matches build.sh, Dockerfile, .gitignore, and the React Router public-asset convention that serves /hp_ssh.wasm and /wasm_exec.js from the app root.
• Switch pnpm react-router build to pnpm build — package.json maps build to react-router build, so this is equivalent and keeps the Nix expression consistent with the documented local build.
• Remove the $PWD -> ../.. sed rewrite — the leftover path substitution is tied to earlier bundled-build layouts and is unnecessary now that the build relies on public-directory assets instead of source-relative paths.

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